1. Field of the Invention
The present invention relates to a silicon annealed wafer having a defect free layer on the surface, which wafer is used for producing semiconductor devices, and also to a silicon epitaxial wafer with a perfect epitaxial layer.
2. Description of the Related Art
A silicon wafer used for producing semiconductor devices is produced by slicing a silicon single crystal grown mainly by the Czochralski method. In the Czochralski method, a silicon single crystal is grown from molten silicon in a quartz crucible, pulling up a seed crystal to solidify the molten silicon.
Normally, oxygen solves into the molten silicon from the raw silicon material and/or the quartz crucible, and the solved oxygen is retained in the single crystal during the silicon solidification. The solubility of oxygen is decreased by the decrease in the temperature of the single crystal, and therefore the single crystal contains oxygen in a saturated state. As a result, oxygen precipitates are generated in the wafer during a heat treatment in the process of manufacturing devices.
Such oxide precipitates or defects induced from the precipitation is called BMD (Bulk Micro Defect). The existence of such BMDs in the surface region of the wafer, on which devices are formed, i.e., on an active region, causes the deterioration of the device characteristics. On the contrary, BMDs existing in a region other than the active region in a substrate suppresses the contamination of the active region into which metal impurities diffuse during the process of manufacturing devices, so that BMD serves as a gettering site for capturing such metal impurities.
In order to effectively use the gettering effect of BMD, a DZ-IG (Denuded Zone-Intrinsic Gettering) treatment is employed. In the DZ-IG treatment, a wafer is subjected to a heat cycle in which the wafer is heated at approximately 1150° C. in an atmosphere containing nitrogen, oxygen or mixture of both, and then annealed at 500° C.-900° C. for several hours or more. Such a heat treatment at a high temperature allows oxygen in the vicinity of the wafer surface to be diffused to the outside thereof. Accordingly, a denuded zone (DZ) having a low oxygen concentration can be formed in the surface layer where neither oxygen precipitates nor defects resulting therefrom reside. Furthermore, the subsequent heat treatment at a lower temperature forms BMDs having the gettering effect in the wafer.
By such DZ-IG treatment, it is possible to obtain a high quality wafer, from which devices having excellent properties can be produced at high yields. However, long process times for the heat treatment and a large variation in the effect of the treatment result in an increase of the production cost.
In most cases, moreover, COP (Crystal Originated Particle) defects (which are called IR-light scattering tomography defects when they exist in the wafer), which pertain to a type of grown-in defects formed in the course of growing a single crystal, reside in the wafer, and it is not possible to remove such COP defects from the wafer by the DZ-IG treatment. The COP defects have a particle size of 0.1 μm or so and they exist in a single crystal at a density of 105-106 counts/cm3 or so. A main factor of deteriorating the device characteristics results from the COP defects in the current process of manufacturing semiconductor devices.
In order to reduce the number of COP defects, a wafer is heated at a high temperature of 1200° C. or so in a non-oxidizing atmosphere containing hydrogen, argon or the like. In this method, a heat treatment at a high temperature is applied to the wafer so as to reduce the density of COP defects in the surface layer of the wafer as well as to diffuse oxygen to the outside thereof. Accordingly, such a heat treatment at a high temperature makes it possible to easily manufacture a high quality wafer including neither oxygen precipitates nor COP defects at the device active region in the surface layer of the wafer.
For instance, Japanese Patent Application Publication No. 10-98047 discloses a technology, in which a wafer is annealed at 1000° C. or more for one hour or more in an atmosphere containing a rare gas or a gas which is selectable from a group of oxygen, nitrogen, a mixture of oxygen and nitrogen, and hydrogen.
In this case, a single crystal, from which wafers are prepared, is produced either under conditions that the oxygen concentration is 4×1017 atoms/cm3 or more and it is maintained within a temperature range of 850-1100° C. for less than 80 minutes during the cooling period for the single crystal growth, or under the condition that it has the nitrogen concentration of at least 1×1014 atoms/cm3. An increased cooling rate within the above temperature range during the single crystal growth causes the size of the COP defects to be decreased, and thereby the density of the defects resulting from the annealing can be effectively reduced.
In the above Japanese Patent Application Publication No. 10-98047, it is shown that a defect free layer can easily be formed on the surface of a wafer. However, no description is given regarding the BMD providing the gettering effect within the wafer. While it has been already known that the doping of nitrogen causes strengthening a silicon crystal, the doped nitrogen influences the generation not only of grown-in defects, but also of OSFs (Oxidation Induced Stacking Faults) or the like resulting from oxygen precipitation or the heat treatment for high temperature oxidation.
In a technical literature (K. Nakai: “Nitrogen and Carbon Effect on the Formation of Grown-in Defects and Oxygen Precipitation Behavior”, The 52th Conference of Japanese Association for Crystal Growth, Bulk Growth Section Meeting, Feb. 8, 2000, pp. 6-9), it is shown that the nitrogen concentration strongly influences the oxygen precipitation. In the case of a single crystal wafer having a diameter of 200 mm, with the same oxygen concentration, an annular potential region of OSF occurrence is formed in a peripheral section of a 50 mm radial width at a doped nitrogen concentration of 5×1014 atoms/cm3, and OSFs are formed over the entire surface area of the wafer at a doped nitrogen concentration of 3×1015 atoms/cm3.
In Japanese Patent Application Publication No. 11-189493, a technology of enhancing the gettering effect for an epitaxial wafer is disclosed. In the technology disclosed therein, the formation of an epitaxial layer on a potential region of OSF occurrence on the surface of a wafer makes it impossible to eliminate oxygen precipitate nuclei even at a high temperature during the epitaxial layer formation, and therefore the nuclei serve as an effective gettering site.
Moreover, Japanese Patent Application Publication No. 11-189493 also describes that nitrogen doping at a concentration of 1×1013 atoms/cm3 or more is normally effective for expanding the width of the annular potential region of OSF occurrence with respect to the center of the wafer, i.e., the axis of pulling the single crystal, to expand the region over the entire wafer.
Furthermore, Japanese Patent Application Publication No. 2001-199795 describes a manufacturing method, wherein, when the passing period through a temperature range of 1100° C. to 700° C. is set to within 200 minutes or less in the process of growing a single crystal at a doped nitrogen concentration of 1×1013-1.2×1015 atoms/cm3, heterogeneity of the BMD density within the wafer resulting from the OSF ring can be eliminated.
As described above, regarding an annealed wafer free from such defects as oxygen precipitates and COPs in the device active region of the wafer surface layer, and which has BMD gettering sites in the inside, various methods have been proposed for decreasing the size of the COP defects by nitrogen dope, so small as to be eliminated by the annealing and at the same time for destributing BMDs sufficiently and uniformly.
However, due to the high segregation coefficient of nitrogen, it is difficult to distribute nitrogen concentration uniformly over the entire single crystal to be grown. Consequently, if it is necessary to dope nitrogen in the single crystal at a high concentration, the crystal area at such high concentration is restricted and no satisfactory yield can be obtained, thereby making it difficult to apply this method to the practical usage.